This invention relates to an improvement in methods of and apparatus for controlling the sewing rate in sewing machines.
It is generally known that there are sewing machines in which stitches are formed on a work piece to be sewn by a needle. The workpiece is movable horizontally by one or more pulse motors. The needle is reciprocated in a vertical plane by another pulse motor. In these conventional sewing machines, when the work is fed by a predetermined amount, there is a tendency for the work to be overshot (overfed) and this overshooting is then eliminated after a lapse of a certain time to attain the predetermined work feed amount. This is due to the characteristics of pulse motors.
One example of conventional sewing machine of this type is shown in FIG. 6. In such a machine although the reciprocally vertical movement rate of the needle is variable, during one cycle wherein the needle moves downwardly from the upper dead center to the bottom dead center and then upwardly from the bottom dead center to the upper dead center, the movement rate of the needle is constant, that is, the needle moves at the same rate throughout the cycle.
However, since the movement rate of the needle is constant throughout one cycle in the conventional sewing machine, any possibility of increase in the sewing rate of a machine is subject to limitation. If the movement rate of the needle is increased, there are the problems in that the needle tends to bend or break while piercing through the work, and/or skip sewing may occur.